Swimming at Stinson Beach in Marin County was declared off-limits through Saturday after lifeguards spotted a great white shark, which might have been lured to the area by a dead whale on the beach, authorities said Tuesday.

The shark was first seen attacking a seal about 1:40 p.m. Monday about 300 yards offshore, said supervising lifeguard Patrick Burns.

Lifeguards then saw "lots of shark activity" in the area near a young fin whale that had beached itself and died earlier in the day, Burns said.

As a necropsy was performed on the whale, blood and oil seeped into the ocean. Burns said it was possible that the sharks were drawn by the blood in the water, though great white sharks are somewhat common in the area around Stinson Beach.

The closest shark was sighted about 5 p.m. Monday in roughly waist-deep water, which is unusually close, Burns said.

If lifeguards see another shark Tuesday, the beach closure will be extended another day. The closure will be lifted after five days with no sightings.